The present invention relates to electronic ballasts and lighting fixtures using an electronic ballast.
Electronic ballasts for lighting a discharge lamp at a high frequency are well known in the art (see e.g., Japanese Unexamined Patent Publication No. 2007-17293). FIG. 10 is a circuit diagram of a one embodiment of an electronic ballast in which a half-bridge type inverter circuit 1 having two switching elements Q1, Q2 is provided and a series circuit formed of the switching elements Q1, Q2 is connected between both ends of a DC power source Vdc. A series resonant circuit 2 including a resonant inductor T1 and a capacitor C1 is connected between a connection point of the switching elements Q1, Q2 and a ground GND of the DC power source Vdc.
A discharge lamp FL as a load is connected between both ends of the resonant capacitor C1 via a resonant and DC blocking capacitor C2. One filament F1 of the discharge lamp FL is connected to a preheating circuit 3 including a serial circuit of an inductor L1 and a capacitor C3 and a preheating source n1, and the other filament F2 is connected to the preheating circuit 3 including a serial circuit of an inductor L2 and a capacitor C4 and a preheating source n2. The preheating sources n1, n2 are set to have the same operating frequency.
In the electronic ballast of this embodiment, when a dimming signal from a dimming circuit 8 is inputted to a frequency control circuit 5, the frequency control circuit 5 determines the operating frequency of the switching elements Q1, Q2 and the switching elements Q1, Q2 are alternately turned on/off at the determined operating frequency by a driving circuit 6. By converting a DC voltage of the DC power source Vdc into a high-frequency voltage by alternately turning on/off the switching elements Q1, Q2 and passing an alternating current through the discharge lamp FL, the discharge lamp FL is lighted at the high frequency. The resonant circuit 2 including the inductor T1 and the capacitors C1, C2 is connected in the path to the discharge lamp FL and energy fed to the discharge lamp FL can be adjusted according to a relationship between the operating frequency of the switching elements Q1, Q2 and a resonant frequency of the resonant circuit 2.
A DC component detecting circuit 7 is connected to the discharge lamp FL such that when a positive or negative DC voltage component is present in the discharge lamp FL, an output signal corresponding to the DC voltage is outputted to a voltage comparator EL. In the case where the voltage comparator EL outputs a low signal, the inverter circuit 1 continues its operation and in the case where the voltage comparator EL outputs a high signal, the output of the inverter circuit 1 is reduced or stopped by controlling the operating frequency of the switching elements Q1, Q2.
When the discharge lamp FL reaches an end of life (EOL) state and an emitter (emissive material) of one filament F1 (or filament F2) is depleted, causing a half-wave discharge state (so-called emission-less state), a DC voltage component occurs in the discharge lamp FL and the DC component detecting circuit 7 outputs an output signal corresponding to the DC voltage component. Then, the output signal from the DC component detecting circuit 7 is inputted to the voltage comparator EL and when this value exceeds a reference value Vref, the voltage comparator EL outputs a high signal and the output of the inverter circuit 1 is reduced or stopped for circuit protection.
In the EOL detection and protection circuits of FIG. 10, even if the operating frequencies of the preheating sources n1, n2 are the same, the resonant frequency of the preheating circuits 3 varies due to variation in the inductors L1, L2 and the capacitors C3, C4. For this reason, a phase difference between continuous preheating currents in the filament F1, F2 in the case where a dimming level changes is generated, resulting in deviation of hot spot positions on the filaments F1, F2. Since a DC voltage component is generated in the high-frequency voltage occurring in the discharge lamp FL due to the deviation of hot spot positions of the filaments F1, F2, even when the lamp at end of life is not connected, it can be erroneously detected that the discharge lamp is at the end of life due to the DC voltage component and a circuit protection function is performed.
Thus, in this example the following method is used to prevent such a malfunction. When the dimming signal for changing the operating frequency is inputted from the dimming circuit 8 to the frequency control circuit 5, a dimming signal detecting circuit 9 detects a change in the dimming signal and outputs a detecting signal to a timer circuit 11. When the timer circuit 11 receives the signal from the dimming signal detecting circuit 9, the timer circuit 11 outputs an ON signal for turning on a switch SW1 (for example, transistor) to the driving circuit 10 for a predetermined time to turn on the switch SW1 for the predetermined time. By turning on the switch SW1, a signal from the DC component detecting circuit 7 is fixed at a low level for the predetermined time.
FIG. 11 shows a timing chart in this example. Since the dimming signal detecting circuit 9 does not detect a change in the dimming signal in the case where the dimming level is not changed, the switch SW1 is not turned on. For this reason, since the signal from the DC component detecting circuit 7 is inputted to the voltage comparator EL as it is, EOL circuit protection is possible in the case where the discharge lamp FL at the end of life is connected.
On the contrary, when a change in the dimming level is rapid, after the change in the dimming signal is completed, the DC voltage component of the discharge lamp FL can be inputted to the voltage comparator EL depending on a time constant of the DC component detecting circuit 7. However, in the case where a delay time of the timer circuit 11 is sufficiently longer than the time constant of the DC component detecting circuit 7, since the timer circuit 11 outputs the ON signal to the driving circuit 10 even when the DC voltage component of the discharge lamp FL is inputted after the change in the dimming signal, the above-mentioned detection error can be prevented.
The electronic ballast disclosed in Japanese Unexamined Patent Publication No. 2007-17293 prohibits operation of the DC component detecting circuit 7 for a predetermined time in the case where the dimming level changes, thereby preventing EOL detection errors due to the DC voltage component caused during a change in the dimming level.
However, when the dimming level changes, the DC voltage component is not necessarily generated and when the dimming level is moderately changed, the DC voltage component is not generated. Accordingly, in the case where a change in the dimming level is relatively small, the dimming level is minutely changed by using a brightness sensor or the like and the dimming level changes due to an external noise, the DC voltage component is not generated. However, since the above-mentioned electronic ballast prohibits a detecting operation of the DC component detecting circuit 7 at change in the dimming level, there are cases where the DC voltage component occurring at the end of life of the discharge lamp FL cannot be detected and thus, a circuit cannot be protected.
TABLE 1Dimming level [%]Maximum peak value [V]Number of sets90-801.49280-701.59370-601.84460-501.81750-401.90940-301.40630-200.782
FIG. 12 and Table 1 show measurement results of the DC voltage component generated at both ends of the discharge lamp in the case where the dimming level is changed, in a dual lamp serial lighting-type electronic ballast. A peak value of the DC voltage component occurring in the discharge lamp in the case where the dimming level is changed from Dim lighting (25% dimmed lighting) to Full lighting (100% lighting) at about 300 ms and a dimming level at the peak value are measured by changing a connecting direction of the discharge lamp. The measurement results are shown for each dimming level and a peak value and N pieces of data in the case where a largest DC voltage component is superimposed at each dimming level are shown.
From FIG. 12 and Table 1, the magnitude of the DC voltage component and a dimming level at which the DC voltage component is superimposed vary depending on variation in the individual discharge lamps and the connecting direction. In this measurement, 66 discharge lamps of FHF24SEN type, FHF24SEW type and FHF24SEL type (33 sets) are used to make measurement at normal temperature and humidity.
In this example, a time during which operation of the DC component detecting circuit 7 is prohibited depends on the type of the discharge lamp and the time constant of the DC component detecting circuit 7. However, as described above, even in the same type of discharge lamps, the magnitude of the DC voltage component occurring in the discharge lamp varies depending on variation in characteristics and environmental conditions such as the speed of the change in dimming level, the number of lamps, the connecting direction and temperature, and timing of occurrence of the DC voltage component and duration when a voltage value exceeds the reference voltage value vary. Accordingly, in this example, it is necessary to set duration when operation of the DC component detecting circuit 7 is prohibited to be sufficiently long. As a result, in the case where the dimming level changes, a protection operation is prohibited for a predetermined time even if the operation of the DC component detecting circuit 7 need not be prohibited. Thus, disadvantageously, a period when the end of life of the discharge lamp FL cannot be detected becomes long.
Furthermore, in this example, to prevent malfunction of the EOL protection circuit at end of life, a control circuit 4 takes a longer time than the change time of the dimming signal by the dimming circuit 8 to change the output of the discharge lamp FL. As a matter of course, when the change time of the dimming signal becomes long, the time to change the output of the discharge lamp FL also becomes longer, resulting in that performance with respect to a dimming operation can be impaired.